Starter Solenoid

March 18, 2022

The advent of start/stop technology for internal combustion engines is straining the lifespan of existing starter solenoids. In a typical combustion engine application, a starter solenoid may undergo approximately 30,000 actuations over a 15-year vehicle lifespan. Start/stop technology can increase this figure by a factor of 10 or more. The drive toward lightweighting results in lower series resistance, which increases inrush currents. Together, these factors push the upper limits of the copper contactors used in existing starter solenoids.

The lower series resistance, resulting from lightweighting, can mean that inrush current is much higher than anticipated given the amount of torque required to turn over an engine. In one example, the inrush current for a 1.3-liter ICE reached 900 A—50% higher than what was expected for the starting system. These high inrush currents, combined with the increase in starter solenoid actuation cycles, due to start/stop technology, cause traditional copper starter solenoid contactors to wear out prematurely. The electrical arcing resulting from high inrush current yields two predominant failure modes: debris (mostly melted copper particles) can mechanically jam the actuator, or the contact wear can become so severe that the contactor can no longer close. During testing, copper contactors weighing between 350 g and 700 g were unable to reach 30,000 cycles at 950 A inrush current.

In one specific case, a manufacturer needed a starter solenoid solution to replace an incumbent part weighing 120 g. The new starter solenoid needed to be capable of 30,000 cycles at 950 A inrush current, achieve 175 A steady-state current handling, and be able to withstand 1 m water immersion for five minutes. The incumbent part, which utilized copper contactors, failed at inrush currents of 600 A and had no immersion protection. The key to meeting the required specifications was to replace pure copper contacts with composite contacts using a proprietary silver-based mixture to dramatically reduce contact ablation and welding.

Performance Evaluation

Evaluating the performance of the composite contactor material involved rapid make/break testing. The closing (make) current was held at a constant 950 A, and the opening (break) current was held at 180 A. Under these conditions, solenoid actuators using copper contacts failed between 10,000 and 27,000 cycles due to debris generated by contact wear. In comparison, substituting composite contacts in the same solenoid actuator resulted in a lifespan of approximately 80,000 cycles. With the composite contacts, failure was due to mechanical wear of the actuator itself. The addition of high-lubricity materials increased the actuator’s lifespan to 200,000 cycles. Contact wear for the composite material at 80,000 cycles was only a fraction of that observed for the copper contacts. The expected lifespan of the composite contacts, based on observed wear, would be greater than 250,000 cycles.

The final design of the new actuator implemented the composite contactor and high-lubricity materials. During testing, contact wear was minimal at 30,000 cycles, and contact debris in the actuator portion of the relay was nearly absent and in no way impeded the operation of the actuator. The actuator design incorporated a high-performance geometry that kept the coil size small and reduced the overall mass of the part. Environmental survivability, including water immersion, was achieved by using an advanced plastic joining process to seal the joint between the contactor and actuator during assembly and by implementing an O-ring seal on the egress points of the stud terminals. The mass of the final production part was 160 g and met all the target performance requirements. The current-handling performance met or exceeded the specifications for contactors weighing more than four times as much.

This article was originally published by Engine + Powertrain Technology International in March 2022

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